/**
 * Mega pixel image rendering library for iOS6 Safari
 *
 * Fixes iOS6 Safari's image file rendering issue for large size image (over mega-pixel),
 * which causes unexpected subsampling when drawing it in canvas.
 * By using this library, you can safely render the image with proper stretching.
 *
 * Copyright (c) 2012 Shinichi Tomita <shinichi.tomita@gmail.com>
 * Released under the MIT license
 */
(function () {

    /**
     * Detect subsampling in loaded image.
     * In iOS, larger images than 2M pixels may be subsampled in rendering.
     */
    function detectSubsampling (img) {
        var iw = img.naturalWidth, ih = img.naturalHeight;
        if (iw * ih > 1024 * 1024) { // subsampling may happen over megapixel image
            var canvas   = document.createElement('canvas');
            canvas.width = canvas.height = 1;
            var ctx = canvas.getContext('2d');
            ctx.drawImage(img, -iw + 1, 0);
            // subsampled image becomes half smaller in rendering size.
            // check alpha channel value to confirm image is covering edge pixel or not.
            // if alpha value is 0 image is not covering, hence subsampled.
            return ctx.getImageData(0, 0, 1, 1).data[3] === 0;
        } else {
            return false;
        }
    }

    /**
     * Detecting vertical squash in loaded image.
     * Fixes a bug which squash image vertically while drawing into canvas for some images.
     */
    function detectVerticalSquash (img, iw, ih) {
        var canvas    = document.createElement('canvas');
        canvas.width  = 1;
        canvas.height = ih;
        var ctx       = canvas.getContext('2d');
        ctx.drawImage(img, 0, 0);
        var data      = ctx.getImageData(0, 0, 1, ih).data;
        // search image edge pixel position in case it is squashed vertically.
        var sy = 0;
        var ey = ih;
        var py = ih;
        while (py > sy) {
            var alpha = data[(py - 1) * 4 + 3];
            if (alpha === 0) {
                ey = py;
            } else {
                sy = py;
            }
            py = (ey + sy) >> 1;
        }
        var ratio = (py / ih);
        return (ratio === 0) ? 1 : ratio;
    }

    /**
     * Rendering image element (with resizing) and get its data URL
     */
    function renderImageToDataURL (img, options, doSquash) {
        var canvas = document.createElement('canvas');
        renderImageToCanvas(img, canvas, options, doSquash);
        return canvas.toDataURL("image/jpeg", options.quality || 0.8);
    }

    /**
     * Rendering image element (with resizing) into the canvas element
     */
    function renderImageToCanvas (img, canvas, options, doSquash) {
        var iw         = img.naturalWidth, ih = img.naturalHeight;
        var width      = options.width, height = options.height;
        var ctx        = canvas.getContext('2d');
        ctx.save();
        transformCoordinate(canvas, ctx, width, height, options.orientation);
        var subsampled = detectSubsampling(img);
        if (subsampled) {
            iw /= 2;
            ih /= 2;
        }
        var d = 1024; // size of tiling canvas
        var tmpCanvas   = document.createElement('canvas');
        tmpCanvas.width = tmpCanvas.height = d;
        var tmpCtx          = tmpCanvas.getContext('2d');
        var vertSquashRatio = doSquash ? detectVerticalSquash(img, iw, ih) : 1;
        var dw              = Math.ceil(d * width / iw);
        var dh              = Math.ceil(d * height / ih / vertSquashRatio);
        var sy              = 0;
        var dy              = 0;
        while (sy < ih) {
            var sx = 0;
            var dx = 0;
            while (sx < iw) {
                tmpCtx.clearRect(0, 0, d, d);
                tmpCtx.drawImage(img, -sx, -sy);
                ctx.drawImage(tmpCanvas, 0, 0, d, d, dx, dy, dw, dh);
                sx += d;
                dx += dw;
            }
            sy += d;
            dy += dh;
        }
        ctx.restore();
        tmpCanvas           = tmpCtx = null;
    }

    /**
     * Transform canvas coordination according to specified frame size and orientation
     * Orientation value is from EXIF tag
     */
    function transformCoordinate (canvas, ctx, width, height, orientation) {
        switch (orientation) {
            case 5:
            case 6:
            case 7:
            case 8:
                canvas.width  = height;
                canvas.height = width;
                break;
            default:
                canvas.width  = width;
                canvas.height = height;
        }
        switch (orientation) {
            case 2:
                // horizontal flip
                ctx.translate(width, 0);
                ctx.scale(-1, 1);
                break;
            case 3:
                // 180 rotate left
                ctx.translate(width, height);
                ctx.rotate(Math.PI);
                break;
            case 4:
                // vertical flip
                ctx.translate(0, height);
                ctx.scale(1, -1);
                break;
            case 5:
                // vertical flip + 90 rotate right
                ctx.rotate(0.5 * Math.PI);
                ctx.scale(1, -1);
                break;
            case 6:
                // 90 rotate right
                ctx.rotate(0.5 * Math.PI);
                ctx.translate(0, -height);
                break;
            case 7:
                // horizontal flip + 90 rotate right
                ctx.rotate(0.5 * Math.PI);
                ctx.translate(width, -height);
                ctx.scale(-1, 1);
                break;
            case 8:
                // 90 rotate left
                ctx.rotate(-0.5 * Math.PI);
                ctx.translate(-width, 0);
                break;
            default:
                break;
        }
    }


    /**
     * MegaPixImage class
     */
    function MegaPixImage (srcImage) {
        if (window.Blob && srcImage instanceof Blob) {
            var img = new Image();
            var URL = window.URL && window.URL.createObjectURL ? window.URL :
                window.webkitURL && window.webkitURL.createObjectURL ? window.webkitURL :
                    null;
            if (!URL) {
                throw Error("No createObjectURL function found to create blob url");
            }
            img.src   = URL.createObjectURL(srcImage);
            this.blob = srcImage;
            srcImage  = img;
        }
        if (!srcImage.naturalWidth && !srcImage.naturalHeight) {
            var _this               = this;
            srcImage.onload         = function () {
                var listeners = _this.imageLoadListeners;
                if (listeners) {
                    _this.imageLoadListeners = null;
                    for (var i = 0, len = listeners.length; i < len; i++) {
                        listeners[i]();
                    }
                }
            };
            this.imageLoadListeners = [];
        }
        this.srcImage = srcImage;
    }

    /**
     * Rendering megapix image into specified target element
     */
    MegaPixImage.prototype.render = function (target, options, callback) {
        if (this.imageLoadListeners) {
            var _this = this;
            this.imageLoadListeners.push(function () {
                _this.render(target, options, callback);
            });
            return;
        }
        options       = options || {};
        var srcImage  = this.srcImage,
            src       = srcImage.src,
            srcLength = src.length,
            imgWidth  = srcImage.naturalWidth, imgHeight = srcImage.naturalHeight,
            width     = options.width, height = options.height,
            maxWidth  = options.maxWidth, maxHeight = options.maxHeight,
            doSquash  = this.blob && this.blob.type === 'image/jpeg' ||
                src.indexOf('data:image/jpeg') === 0 ||
                src.indexOf('.jpg') === srcLength - 4 ||
                src.indexOf('.jpeg') === srcLength - 5;
        if (width && !height) {
            height = (imgHeight * width / imgWidth) << 0;
        } else if (height && !width) {
            width = (imgWidth * height / imgHeight) << 0;
        } else {
            width  = imgWidth;
            height = imgHeight;
        }
        if (maxWidth && width > maxWidth) {
            width  = maxWidth;
            height = (imgHeight * width / imgWidth) << 0;
        }
        if (maxHeight && height > maxHeight) {
            height = maxHeight;
            width  = (imgWidth * height / imgHeight) << 0;
        }
        var opt = {width: width, height: height};
        for (var k in options) opt[k] = options[k];

        var tagName = target.tagName.toLowerCase();
        if (tagName === 'img') {
            target.src = renderImageToDataURL(this.srcImage, opt, doSquash);
        } else if (tagName === 'canvas') {
            renderImageToCanvas(this.srcImage, target, opt, doSquash);
        }
        if (typeof this.onrender === 'function') {
            this.onrender(target);
        }
        if (callback) {
            callback();
        }
    };

    /**
     * Export class to global
     */
    if (typeof define === 'function' && define.amd) {
        define([], function () {
            return MegaPixImage;
        }); // for AMD loader
    } else {
        this.MegaPixImage = MegaPixImage;
    }

})();
